There is a long standing need for improved systems for measuring the position and motion of one object relative to another or one part of an apparatus relative to another. For example, modern active automotive suspension systems require some means for detecting the relative position and motion, e.g. height, velocity and acceleration, of the vehicle running gear (wheels, axles, etc.) with respect to the vehicle frame as the vehicle travels. This has historically been done, for example, by attaching the case of a potentiometer to the running gear and the wiper arm to the vehicle frame or vice versa. From this an electrical signal may be derived whose magnitude is proportional to the instantaneous relative position of the running gear and the frame. By differentiating the signal, the velocity and acceleration of the running gear relative to the frame may also be determined. Ultrasonic detectors which measure the height of the vehicle frame or body above the road have also been used for the same purpose.
A common difficulty with these position and/or motion detectors is that they are difficult to protect from the adverse ambient in which they must function. When used on vehicles, for examples, they are exposed to dirt, dust, moisture, salt and many other highly corrosive or damaging ambients. Many of the prior art sensors are not hermetic, e.g., they have moving seals which tend to wear out or suffer from other limitations well known in the art. Thus, a need continues to exist for improved position/motion sensors, especially for sensors which are hermetic, i.e., easily rendered substantially impervious to dirt, dust, moisture and the like.